Catalytic reforming is well known in the petroleum industry and refers to the treatment of naphtha fractions to improve the octane rating by the production of aromatics. The more important hydrocarbon reactions occurring during reforming include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, and dehydrocyclization of acyclic hydrocarbons to aromatics. A number of other reactions also occur, including the following: dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butane. Hydrocracking reactions are to be particularly minimized during reforming as they decrease the yield of gasoline boiling products and hydrogen.
Because of the demand for high octane gasoline for use as motor fuels, etc., and of the demand for benzene as a chemical reactant extensive research is being devoted to the development of improved reforming catalysts, improved catalysts for benzene production, catalytic reforming processes and benzene production processes. Catalysts for successful reforming processes must possess good selectivity. That is, they must be able to produce high yields of liquid products in the gasoline boiling range which contain large concentrations of high octane aromatic hydrocarbons and low concentrations of light gaseous hydrocarbons. Benzene production catalysts must also have good selectivity to produce high yields of benzene and low yields of light gaseous hydrocarbons. Also, the catalysts should possess good activity in order that the temperature required to produce a certain quality product need not be too high. It is also necessary that catalysts either possess good stability in order that the activity and selectivity characteristics can be retained during prolonged periods of operation, or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Reforming catalysts and benzene production catalysts are usually composed of a highly dispersed transition metal(s) on a metal oxide support. Typically, the transition metal is a noble metal, most notably platinum. However, there are numerous metal oxide supports. Examples are: silica, alumina, and a plethora of natural and man-made zeolites. Intermediate pore size zeolites are one sub-class of the zeolites.
By "intermediate pore size", as used herein, is meant an effective pore aperture in the range of about 5 to 6.5 .ANG. when the molecular sieve is in the H-form. Molecular sieves having pore apertures in this range tend to have unique molecular sieving characteristics. Unlike small pore zeolites such as erionite and chabazite, they will allow hydrocarbons having some branching into the molecular sieve void spaces. Unlike larger pore zeolites such as the faujasites and mordenites, they can differentiate between n-alkanes and slightly branched alkanes on the one hand and larger branched alkanes having, for example, quaternary carbon atoms.
The effective pore size of the molecular sieves can be measured using standard adsorption techniques and hydrocarbonaceous compounds of known minimum kinetic diameters. See Breck, Zeolite Molecular Sieves, 1974 (especially Chapter 8) and Anderson, et al., J. Catalysis 58, 114 (1979), both of which are incorporated by reference.
Intermediate pore size molecular sieves in the H-form will typically admit molecules having kinetic diameters of 5.0 to 6.5 .ANG. with little hindrance. Examples of such compounds (and their kinetic diameters in .ANG.) are: n-hexane (4.3), 3-methylpentane (5.5), benzene (5.85), and toluene (5.8). Compounds having kinetic diameters of about 6 to 6.5 .ANG. can be admitted into the pores, depending on the particular sieve, but do not penetrate as quickly and in some cases are effectively excluded. Compounds having kinetic diameters in the range of 6 to 6.5 .ANG. include: cyclohexane (6.0), 2,3-dimethylbutane (6.1), 2,2-dimethylbutane (6.2), m-xylene (6.1) and 1,2,3,4-tetramethylbenzene (6.4). Generally, compounds having kinetic diameters of greater than about 6.5% do not penetrate the pore apertures and thus are not absorbed into the interior of the molecular sieve lattice. Examples of such larger compounds include: o-xylene (6.8), hexamethylbenzene (7.1), 1,3,5-trimethylbenzene (7.5), and tributylamine (8.1).
The preferred effective pore size range is from about 5.3 to about 6.2 .ANG.. Among the materials falling within this range are the crystalline silica polymorph, silicalite, RE 29,948 organosilicates, and the chromia silicate CZM.
In performing adsorption measurements to determine pore size, standard techniques are used. It is convenient to consider a particular molecule as excluded if it does not reach at least 95% of its equilibrium adsorption value on the zeolite in less than about 10 minutes (p/po=0.5; 25.degree. C.).
By "crystalline silica polymorphs", as used herein, is meant materials having very low aluminum contents (or high silica:alumina mole ratios). Aluminum contents of these materials are generally less than about 4000 ppm, preferably less than about 2000 ppm, more preferably less than about 1000 ppm.
Intermediate pore size crystalline silica polymorphs useful in the present invention include silicalite, as disclosed in U.S. Pat. No. 4,061,724, and the "U.S. Pat. No. 29,948 organosilicates", disclosed in U.S. Pat. No. 29,948, both of which are incorporated by reference. The essentially alumina-free chromia silicate, CZM, is disclosed in U.S. application Ser. No. 160,618, Miller, filed June 28, 1980, incorporated by reference.
Intermediate pore size zeolites include materials such as CZH-5 and members of the ZSM series, e.g., ZSM-5, ZSM-11, ZSM-12, ZSM-21, ZSM-22, ZSM-23, ZSM-35, and ZSM-38. ZSM-5 is described in U.S. Pat. Nos. 3,702,886, 29,948 and 3,770,614; ZSM-11is described in U.S. Pat. No. 3,709,979 (See also, Bibby, et al., Nature, 280, 664-665 (Aug. 23, 1980) which reports the preparation of a crystalline silicate called "silicalite-2"); ZSM-12 is described in U.S. Pat. No. 3,832,449; ZSM-21 and ZSM-38 are described in U.S. Pat. No. 3,948,758; ZSM-22 is described in U.S. Pat. Nos. 4,481,177 and 4,556,477; ZSM-23 is described in U.S. Pat. No. 4,076,842; ZSM-35 is described in U.S. Pat. No. 4,016,245; ZSM-38 is described in U.S. Pat. No. 4,046,859; ZSM-48 is described in U.S. Pat. No. 4,397,827 and CZH-5 is disclosed in U.S. application Ser. No. 166,863, Hickson, filed Jul. 7, 1980. All of these patents, publications and specifications which have not previously already been incorporated herein by reference are hereby so incorporated. The intermediate pore size zeolites can include "crystalline admixtures" which are thought to be the result of faults occurring within the crystal or crystallite area during the synthesis of the zeolites. The "crystalline admixtures" are themselves zeolites but have characteristics in common, in a uniform or nonuniform manner, to what the literature reports as distinct zeolites. Examples of crystalline admixtures of ZSM-5 and ZSM-11 are disclosed and claimed in U.S. Pat. No. 4,229,424, Kokotailo, Oct. 21, 1980 (incorporated by reference). The crystalline admixtures are themselves intermediate pore size zeolites and are not to be confused with physical admixtures of zeolites in which distinct crystals or crystallites of different zeolites are physically present in the same catalyst or hydrothermal reaction mixture.
Additionally, zeolites SSZ-20 and SSZ-23 are preferred catalysts. SSZ-20 is disclosed in U.S. Pat. No. 4,483,835, and SSZ-23 is disclosed in U.S. Pat. No. 4,859,442, both of which are incorporated herein by reference.
The crystalline silicate may be in the form of a borosilicate, where boron replaces at least a portion of the aluminum of the more typical aluminosilicate form of the silicate. Borosilicates are described in U.S. Pat. Nos. 4,268,420; 4,269,813; 4,327,236 to Klotz, the disclosures of which patents are incorporated herein, particularly that disclosures related to borosilicate preparation.
In the borosilicate, the preferred crystalline structure is that of ZSM-5, in terms of X-ray diffraction pattern. Boron in the ZSM-5 type borosilicates takes the place of aluminum that is present in the more typical ZSM-5 crystalline aluminosilicate structures. Borosilicates contain boron in place of aluminum, but generally there are some trace amounts of aluminum present in crystalline borosilicates.
Still further crystalline silicates which can be used in the present invention are iron silicates and gallium silicates.
Borosilicates and aluminosilicates are the more preferred silicates for use in the present invention. Aluminosilicates are the most preferred.
Silicalite is an intermediate pore zeolite and is generally considered to be a ZSM-5 zeolite which has a high silica:alumina (SiO.sub.2 :A12O.sub.3) ratio. Examples of its methods of manufacture can be shown in: Dwyer, et al, U.S. Pat. Nos. 3,941,871, issued Mar. 2, 1976 and 4,441,991, issued Apr. 10, 1984; and Derouane, et al, EPO Application No. 186,479, published Feb. 7, 1986, all of which are incorporated by reference in their entirety.
Dwyer, et al suggest that a platinum-loaded silicalite catalyst can be used in reforming hydrocarbons, as well as other types of reactions. The process conditions in Dwyer, et al are listed as: a temperature between 700.degree. F. and 1000.degree. F.; a pressure between 100 psig and 1000 psig (preferably 200-700 psig); a liquid hourly space velocity (LHSV) between 0.1 and 10 (preferably between 0.5 and 4); and a hydrogen to hydrocarbon (H.sub.2 /HC) mole ratio between 1 and 20 (preferably 4 and 12). Detz, et al, U.S. Pat. No. 4,347,394, issued Aug. 31, 1982, disclose a process for selectively producing benzene using a catalyst having platinum on an intermediate pore zeolite which is substantially free of acidity (such as silicalite). The process conditions can be: temperatures greater than 480.degree. C. (more preferably at relatively high temperatures, such as between 510.degree. C. and 595.degree. C.; pressures of between atmospheric and 10 bar; an LHSV between 0.1to 15; and hydrogen may or may not be added. P. Jacobs, et al, "Comparison of Acid to Metal Catalyzed Conversion of N-decane and Cyclodecane on ZSM-5 and Faujasite-type Zeolites", J. Mol. Cat., 27, 11 (1984) show reacting a platinum silicalite catalyst with the test compounds n-decane and cyclodecane.
Reforming at low pressure in the absence of added hydrogen produces a relatively high liquid yield of relatively high octane reformate. Dehydrogenation and dehydrocyclization to produce benzene at low pressures produces a relatively high liquid yield of relatively high benzene content liquid product. Unfortunately, conventional catalysts foul quickly at these conditions which makes this operation impractical. Accordingly, the need has arisen for reforming and benzene production catalysts which have an acceptable run length under the conditions noted above.